In the field of recent storage systems, a technology for eliminating duplicated data without having a user be aware of the elimination (to be referred to as “de-duplication technology” hereinafter) has been attracting attention. A storage system applied with the de-duplication technology conceals the elimination of duplicated data by disposing a virtualization layer. Therefore, with the application of the de-duplication technology, a user can access user data by means of a conventional access method, and at the same time use a limited data storage area within the storage system with a high degree of efficiency.
A de-duplication system includes a complete matching system and a non-complete matching system. In the complete matching system, for example, an I/O request received from a client is of a block level or a file level. In the case of the block level, when the content of a certain block data matches the content of another block data, the blocks are determined as duplicated data, and one of the blocks is eliminated. In the case of the file level, when the content of a certain file completely matches the content of another file, the files are determined as duplicated data, and one of the files is eliminated (see Patent Literature 1, for example). In the non-complete matching system, on the other hand, an I/O request received from the client is of the file (particularly contents) level. In the case of the contents level, for example, the features of certain contents and of other contents are extracted, and when the degree of similarity between these features is high, these contents are determined as duplicated data (see Patent Literature 2, for example).